The present invention relates to crystalline thermoplastic articles such as fibers or films having high tenacity, modulus and toughness values and a process for their production which includes a gel intermediate.
The preparation of high strength, high modulus polyethylene fibers by growth from dilute solution has been described by U.S. Pat. No. 4,137,394 to Meihuizen et al. (1979) and pending application Ser. No. 225,288 filed Jan. 15, 1981 now U.S. Pat. No. 4,356,138.
Alternative methods to the preparation of high strength fibers have been described in various recent publications of P. Smith, A. J. Pennings and their coworkers. German Off. No. 3004699 to Smith et al. (Aug. 21, 1980) describes a process in which polyethylene is first dissolved in a volatile solvent, the solution is spun and cooled to form a gel filament, and finally the gel filament is simultaneously stretched and dried to form the desired fiber.
UK Patent application GB No. 2,051,667 to P. Smith and P. J. Lemstra (Jan. 21, 1981) discloses a process in which a solution of the polymer is spun and the filaments are drawn at a stretch ratio which is related to the polymer molecular weight, at a drawing temperature such that at the draw ratio used the modulus of the filaments is at least 20 GPa. The application notes that to obtain the high modulus values required, drawing must be performed below the melting point of the polyethylene. The drawing temperature is in general at most 135.degree. C.
Kalb and Pennings in Polymer Bulletin, vol. 1, pp. 879-80 (1979), Polymer, 2584-90 (1980) and Smook et al. in Polymer Bull., vol. 2, pp. 775-83 (1980) describe a process in which the polyethylene is dissolved in a nonvolatile solvent (paraffin oil) and the solution is cooled to room temperature to form a gel. The gel is cut into pieces, fed to an extruder and spun into a gel filament. The gel filament is extracted with hexane to remove the paraffin oil, vacuum dried and then stretched to form the desired fiber.
In the process described by Smook et. al. and Kalb and Pennings, the filaments were non-uniform, were of high porosity and could not be stretched continuously to prepare fibers of indefinite length.